This invention relates to a press-molding apparatus for obtaining an optical element having a predetermined shape by press-molding a preformed glass material after it is heated and softened, and a method of manufacturing the same.
Recently, in a field of production of an optical element such as an optical lens, it is desired to obtain a high-accuracy lens shape without carrying out surface polishing. To this end, proposal has been made of a method comprising the steps of preparing a pre-shaped glass material as a preform, heating and softening the preform, and pressing the preform by the use of a pressing mold having a high-accuracy pressing surface (Japanese Unexamined Patent Publication JP 2001-10829 A).
Referring to FIG. 1, description will be made of a basic structure of a press-molding apparatus for use in pressing of the type described.
In the press-molding apparatus illustrated in FIG. 1, a plurality of (for example, four) preforms are simultaneously pressed by an upper mold 602 and a lower mold 604. Each of the upper mold 602 and the lower mold 604 has an elongated shape extending in a transversal or horizontal direction in the figure. The upper mold 602 and the lower mold 604 have a plurality of molding surfaces 614 and 616 formed on confronting surfaces thereof and arranged in a single line, respectively, to provide preforms with a lens shape. The upper mold 602 and the lower mold 604 are fixed to a pair of upper and lower supporting shafts 610 and 612 via supporting members 606 and 608, respectively.
The upper supporting shaft 610 is fixed to an apparatus frame (not shown) while the lower supporting shaft 612 is driven by a driving mechanism in a vertical direction. The upper mold 602 and the lower mold 604 are surrounded by heating coils 618 and 620 for heating the upper and the lower molds 602 and 604 by high-frequency induction heating, respectively. Between the upper mold 602 and the lower mold 604, the preforms are supplied after heated and softened to a viscosity between 105.5 and 109 poises. The preforms in the softened state is clamped and pressed by the upper mold 602 and the lower mold 604 so that high-accuracy processed surfaces are formed on the preforms.
As a method of producing a plurality of optical elements by press molding a plurality of preforms by a plurality of molding surfaces as described above, disclosure is made of a method in which a plurality of preforms are simultaneously dropped and supplied onto a plurality of molding surfaces of a pressing mold (Japanese Unexamined Patent Publication JP 11-29333 A). However, if a plurality of preforms are dropped and supplied onto the molding surfaces, each of the preforms may not be exactly supplied on the center of the molding surface. If the preform is press-molded in such a state, thickness deviation may be caused.
In order to solve the above-mentioned problem, proposal has been made of guide means for guiding each preform during dropping to supply the preform on the center of each molding surface as exactly as possible (Japanese Unexamined Patent Publication JP 11-29333 mentioned above). As illustrated in FIG. 2, a positioning arm 700 as the guide means is extended in a horizontal direction and is provided with through holes 702 having a funnel-like shape (i.e., an upper part has a greater inner diameter) and arranged in a single line.
The positioning arm 700 is placed on the upper side of the lower mold 604 and serves to guide the preforms (depicted by the reference symbol P in the figure), which are dropped and supplied from an upper position by a transfer apparatus (not shown), onto the molding surfaces 616 through the through holes 702. By the positioning arm 700, the preforms P can be guided to the molding surfaces 616, respectively, without jumping out from the molding surfaces 616.
In the positioning arm illustrated in FIG. 2, however, the preform P may be inclined during dropping through the through hole 702 depending upon the shape or the surface condition of the preform P. In this event, the preform is deviated from the center of the molding surface 616. If the press-molding is carried out in such a state that the preform is deviated in position, the thickness deviation is caused and the surface accuracy is degraded.
In the meantime, Japanese Unexamined Patent Publications JP 9-2825 A and JP 5-97448 A disclose a structure in which a positioning mechanism comprising a large number of lever-like movable members for clamping and positioning a plurality of materials, respectively, is attached to the pressing mold (lower mold). In such a structure that the positioning mechanism is attached to the pressing mold, however, a temperature elevation rate of the pressing mold is lowered because heat capacity of the pressing mold is increased because of presence of the positioning mechanism. In addition, if the positioning mechanism is repeatedly subjected to heating and cooling cycles in the manner similar to the pressing mold, the positioning mechanism tends to be deformed so that the positioning accuracy is lowered.
In a method of press-molding a plurality of preforms introduced into the pressing mold after the preforms are heated and softened to a temperature higher than that of the pressing mold, it is required to simultaneously supply a plurality of preforms into the pressing mold and to simultaneously press mold the preforms so that the preforms are press-molded under the same molding conditions.
In the method disclosed in Japanese Unexamined Patent Publication JP 9-2825 A, a plurality of preforms may be simultaneously corrected in position. However, this publication does not disclose whether or not a plurality of preforms can be simultaneously supplied to the pressing mold and simultaneously press molded.